Industry standards in door geometry dictate lock placement and generally offer contractors two latch and knob axis-to-door-edge distances. Latch and knob axis-to-door-edge distance is described and understood within the trade as “backset”. Backset is the distance from the edge of the door (wood or metal), from which the latch bolt extends, to the center of rotation of the knob. The two common backsets are 60 mm (2⅜ inches) often found in older doors and 70 mm (2¾ inches) found in more modern doors.
In the past, it was common for producers to produce separate lock and latch assemblies for each backset, and retailers would stock both types. However, both retailers who have limited shelf space, and contractors who must purchase and install latch and lock sets have demanded single latch and lock sets that can easily be adjusted to fit either backset.
A modern latch mechanism generally has a tubular latch housing mounted in a lateral bore at the edge of the door. The latching bolt has a canted forward face that provides retraction of the bolt when the face abuts against a striker plate in the doorjamb upon closing the door. The bolt springs back into the hole of the striker plate to latch the door shut. In wood doors the faceplate is conventionally rectangular in shape and fits within a mortised or chiseled recess in the door edge. The rectangular shape may have rounded corners for aesthetic purposes. In metal doors, a circular faceplate is conventionally used which has a plurality of serrations or ribs in its periphery and which is driven into and secured to the lateral bore in the door.
To date, several types of adjustable backset latches have been developed. Attempts to solve the problems associated with two common backsets have included various mechanisms to vary the backset, including latches of various lengths, multiple movable cams, linkages incorporated within the latch, fixed cam/axis devices offering two backset dimensions, moveable cams and linkages incorporated within the knob, and latch housings with multiple settings. However, many are very complex, with many parts. In addition, there is limited space available within a door, and many of the adjustable backset latch devices must have portions removed and replaced in order to change backsets, which requires disassembly of the latch mechanism in order to change backsets. Thus, there is a need for a simple, compact and efficient adjustable backset latch mechanism that can be cost-effectively manufactured, stocked, purchased, and installed with ease without disassembly of the latch mechanism in order to change backset.